The present invention relates generally to the manufacture of display screens for cathode ray tubes, and more particularly to an improved method for making CRT display screens using a virtual mask exposure system. The invention has special utility in the production of phosphor dot screens for shadow mask type color display tubes, particularly screens of the black-surround variety. For convenience, the invention will therefore be described primarily in relation to the manufacture of such screens.
A conventional dot screen type color display tube includes three electron guns arranged in a delta configuration. The three guns project a like number of electron beams through a shadow mask onto a display screen comprising a mosaic pattern of phosphor deposits arranged in a multiplicity of dot triads. Each triad includes a dot of a red-, a green-, and a blue-emitting phosphor. For improved display brightness, the screen may include a matrix layer of light-absorbing material that surrounds and separates the phosphor dot deposits. Such a screen, which has come to be known as a "black surround" screen, is the subject of U.S. Pat. No. 3,146,368 to Fiore et al.
The mosaic phosphor dot pattern of a dot-screen tube usually is formed by a direct photoprinting process in which a screen area on the inner surface of the faceplate is first coated with a photosensitive phosphor slurry. Then, with the shadow mask temporarily mounted on the faceplate, the coating is exposed to light projected through the mask's apertures from a source located at the same relative position as one of the electron guns in an assembled tube. After removing the shadow mask, the coating is treated to remove the unexposed portions, leaving a pattern of dots of one phosphor color. The process is then repeated for each of the remaining colors, with the light source shifted to the appropriate electron gun position for each color. In this manner, a separate triangular group consisting of a red, a green, and a blue phosphor dot is deposited on the faceplate for each aperture in the mask. The prevailing practice is to make the individual phosphor dots smaller in size than the apertures in the shadow mask. This is generally accomplished by exposing the dots through a shadow mask that has apertures of a temporarily smaller size. Then, after the phosphor dots are deposited, the mask is re-etched to enlarge the apertures to a final, larger size. Re-etching of shadow mask apertures is shown in U.S. Pat. No. 2,961,313 to Amdursky, for example. An alternative procedure is to reduce the diameter of the shadow mask holes temporarily by electroplating, as described in U.S. Pat. No. 3,231,380 to Law, or by electrophoretic coating with a non-metallic material, as taught by U.S. Pat. No. 3,070,441 to Schwartz. The size of the phosphor dots also can be made smaller without modifying the shadow mask by very careful control of the light exposure step. See, for example, previously mentioned U.S. Pat. No. 3,146,368.
Black surround screens may be made in a variety of ways, but the usual procedure is to form the light-absorbing matrix layer before depositing the phosphor dots. For example, as described in U.S. Pat. No. 3,558,310 to Mayaud, the screen area of the faceplate is coated first with a photochardenable material, such as dichromate-sensitized polyvinyl alcohol (pva). With the shadow mask mounted in position, the coating is given three separate exposures, one from each electron gun position. The mask is then removed and the unexposed portions of the coating washed off, leaving a pattern of hardened pva dots. The dot pattern is covered with a light-absorbing coating of colloidal graphite, which is dried and then treated with a chemical agent, such as hydrogen peroxide, to remove the pva dots and the overlying portions of the graphite coating. This provides the screen area with a light-absorbing matrix layer having a pattern of openings for receiving the color phosphor dots, which are then deposited as previously described.
Screening methods of the prior art as described have a number of disadvantages. For example, it will be noted that it is necessary to attach the shadow mask to the faceplate several times during the manufacture of a tricolor display tube according to the above-described process--once for the black surround exposure, once for each color exposure, and once prior to final assembly of the tube. Shadow masks can be damaged relatively easily, and once damaged usually cannot be reused. Obviously, the more times a mask must be mounted and removed, the greater the chance it will be damaged. The various means, such as reetching, used to provide different mask aperture sizes at different stages in a tube's manufacture also damage a certain number of shadow masks, leading to lower yields and increased production cost. In addition, unless the shadow mask is accurately repositioned for each exposure, misregistration of the different color phosphor dots with the holes in the black surround layer, or with each other, may result.
Other drawbacks of the prior art processes result because the photosensitive coatings are exposed from the "front", i.e., from the side away from the faceplate surface. Because the photoinsolubilization process begins at the side of the coating nearest the light source and proceeds through the thickness of the layer as the exposure continues, exposure and coating uniformity are critical if well adhered dots of uniform size are to be obtained. Slight underexposure or an overthick coating may result in undersized dots or ones that fail to adhere to the faceplate. Overexposure (or a too thin coating) causes overly large dots with ragged edges.
A general object of the present invention is, therefore, to provide an improved process for screening a color display cathode ray tube that is free from the drawbacks enumerated above.
A more specific object of the invention is to provide a novel method for applying a pattern of uniform, well defined deposits on the faceplate of a cathode ray tube.
Another object of the invention is to provide a method for screening shadow mask color display tubes that minimizes the possibility of mask damage.
Still another object of the invention is to provide an improved screening method in which photosensitive coating and exposure uniformity are less critical than in certain prior art processes.